MEDIUM OPTICAL STORAGE OF DATA AND METHODS FOR ITS MANUFACTURE
DESCRIPTION OF THE INVENTION The invention relates to an optical data storage method comprising at least: a substrate having a surface with data stored in grooves that are engraved on the substrate and in spaces separating the grooves, a reflective layer covering the surface having an intrinsic optical reflectivity R at a wavelength?, a stack of transparent cover that is formed on the reflecting layer, the groove pattern is readable through the cover stack by means of a radiation beam focused that has the wavelength? The invention is further related to methods of manufacturing said medium. The reading of read-only optical means (ROM) is based on the phase modulation of a focused radiation beam, for example a laser beam, when it is reflected on a groove pattern. In the types of digital versatile disc media (DVD) and compact disc (CD) the grooves are duplicated on the substrate and subsequently covered with a thin metal layer. This reflective layer results in a high reflection REF: 177359 signal. Since the data reading is obtained via the substrate side of the disk, the groove shape is well preserved. In new discs of the Blue-ray high-density (BD) ROM type, the ROM information is also duplicated on a substrate, the groove pattern is subsequently covered with a metallic mirror. Next-generation BD discs differ from older discs such as CDs and DVDs in that the data is read from the opposite side of the disc, specifically through a thin-cover cover layer. This results in a slightly damaged groove shape due to non-perfect transfer by the metal layer of the groove shape. The difference between the reading through the substrate and the reading through the cover in the case of grooves that have been duplicated in the substrate is given in Figure 1. In particular, in the case of small channel bit lengths ( CBL) such as on 27GB BD discs, the non-perfect groove shape can lead to increased synchronization undulations. In addition, the procedure range for the 25 GB and 23.3 GB versions of the BD disks may suffer from a non-perfect transfer of the groove shape through the metal reading layer. Three data capacities are prescribed in the BD-ROM standard, specifically 23.3, 25 and 27 GB recorded on a single 12 cm rewritable disc. The smallest groove lengths correspond, for example, to the density of 27, 25 and 23.3 GB are 160, 149 and 138 nm (corresponding to bit lengths of two channels also called 12), respectively. An object of the invention is to provide an optical data storage medium of the kind described in the opening paragraph, with an improved signal reading quality, in a practical reduced synchronization undulation. A further objective is to provide a method for manufacturing said optical data storage medium. According to the invention, this objective is obtained with an optical data storage medium as claimed in claim 1, which is characterized in that the value of R separating the grooves is substantially different from the value of R at the bottom of the furrows. It should be noted that the definition of the reflection value R is such that it is based on material properties of the reflecting layer and not on the effects of optical interference due, for example, as opposed to the phase of the radiation reflected from the bottom of a groove and the surface. A good solution would be an ultra-thin homogenous layer with a high reflection R value. However, such a layer is difficult to carry out. Therefore, it is proposed in a modality of. according to the invention, applying a reflective layer with a pattern that is uniquely or predominantly present on the spaces separating the grooves, also called surfaces, of the duplicated area. In another embodiment, the reflective layer is present only or predominantly at the bottom of the grooves. Preferably, the reflective layer comprises a material having a refractive index nr substantially different from the reflection index nc of the material of the cover stack in order to obtain a sufficient reflection at the boundary between the reflective layer and the cover stack. In one embodiment, the reflective layer is a metallic layer, for example of Ag or Al or other suitable metals and alloys thereof. Since only the spaces separating the furrows from the bottom of the furrows are covered with a reflective layer, amplitude modulation will also be a dominant contribution to furrow detection. The unduplicated part of the disc is covered with the reflective layer, while the duplicated and recorded grooves are observed as holes in this layer or the bottom of the grooves are small mirrors that improve the signal and therefore improve the signal quality. In one modality,? is approximately 405 nm, the grooves are formed in a spiral-shaped track pattern having a track pitch of 0.320 + 0.010 μm, and the length of the grooves in the direction of the track is modulated according to a limited code of Shift length with shift lengths >; 2CBL and < 8CBL where CBL = 80.00 nm +/- 0.07 nm, 74.50 nm +/- 0.07 nm or 69.0 nm +/- 0.07 nm. These parametric values correspond to the BD-ROM format. According to the invention, a further object is obtained with the method as claimed in claim 8, comprising the steps of: providing a substrate having a surface with data stored in the grooves that are recorded within the substrate and in spaces which separate the grooves, provide a reflective layer that covers the surface by deposition of the inclined electrodeposition type, with an angle of inclination such that the reflective layer is deposited predominantly in the surface surface area of the substrate, providing a transparent cover stack that It is formed on the reflecting layer. Said reflective layer with a pattern can be obtained, for example, by deposition of the inclined electrodeposition type. It makes use of what is called the shadow effect. If the incident angle of inclination is greater than about 45 ° (with respect to the normal incidence which is taken to be 0 °), the bottom part of the shorter groove is, for example, a groove 12, and will not be covered by the atoms that bomb. As a result, only the adjacent surfaces are covered with a reflective layer. The bottom of the larger grooves, such as the lengths of 8 runs, ie 18, can be covered by a thin reflecting layer, but this can be done uniformly by rotating the disc during electrodeposition. The thin reflective layer at the bottom has a much lower intrinsic reflection value than the reflective layer of the surface area. Alternatively, an additional objective is obtained by the method as claimed in claim 9, comprising the steps of: a) providing a substrate having a surface with data stored in grooves that are recorded within the substrate and in spaces that separate the grooves, b) provide a layer covering the surface by centrifugal coating in such a way that the layer has a greater thickness in the grooves than in the spaces, c) isotropically engraving the coated layer in a centrifugal manner so that only the The lower part of the groove is covered with the coated layer in a centrifugal manner, d) It provides a transparent covering stack that is formed on the substrate and the layer subjected to centrifugal coating. In one embodiment, the layer of a layer subjected to centrifugal coating comprises a material having a refractive index nr substantially different from the refractive index nc of the material of the cover stack in order to obtain sufficient reflection at the boundary between the reflecting layer and the roof stack. Another embodiment of the method comprises the following steps between step c) and step d) of the method: c ') depositing an additional reflective layer in the spaces separating the grooves on the layer subjected to centrifugal coating covering the lower part of the layers. grooves and on the side walls of the grooves, c ") eliminate the layer applied by centrifugal coating that covers the bottom part of the grooves, which includes the additional reflective layer that covers this layer subjected to centrifugal coating. the material of the additional reflective layer is deposited at the bottom of the grooves Alternatively a reflective layer with a pattern can be provided by means of printing techniques such as, for example, transfer printing or dip coating. Optical data storage means and the method of manufacture will be elucidated in greater detail with reference to the appended figures, which: Figure 1 shows a schematic cross-section of an optical data storage medium according to the prior art to illustrate the difference between the reading through the substrate and the reading through the cover in the case of duplicate grooves in the substrate . Figure 2 shows a schematic cross section of an optical data storage medium according to the invention with a reflective layer with a pattern for improving the reading of a BD-ROM disc. Figure 3 shows the adjustment to perform the step of deposition of a reflective layer covering the surface by deposition of the inclined electrodeposition type of the manufacturing method, according to the invention. Figures 4a-4d show the steps of the manufacturing method according to the invention to obtain a reflective layer covering the bottom of the grooves. In Figure 1 a schematic cross section of an optical data storage is shown illustrating the difference between the reading through the substrate and the reading through the cover in the case of grooves that duplicate in the substrate. The metallic layer acts as a reflecting layer. The shape of the surface between the reflective layer and the cover layer has a somewhat deteriorated groove shape compared to the surface between the reflective layer and the substrate. This is due to a non-perfect transfer of the reflecting layer of the groove shape. In particular, in the case of small grooves representing small channel bit lengths (CBL), such as BD discs of 27 GB, a non-perfect groove shape can generate an increased synchronization ripple. In Figure 2 a schematic cross-section of an optical data storage medium according to the invention is shown. It comprises a substrate, which has a surface with data stored in the grooves that are recorded on the substrate and in the spaces that separate the grooves. A reflective layer covers the surface and has an intrinsic optical reflectivity R for a focused radiation beam, for example the laser beam of an optical pickup unit of a reading device of an optical data storage medium. A transparent roof stack is formed on the reflecting layer. The pattern of the grooves is readable through the roof stack by means of the focused beam of radiation. The value of R in the spaces that separate the grooves is substantially greater than the value of R in the bottom of the grooves. This is due to the fact that the reflecting layer is predominantly present in the spaces that separate the grooves. The reflective layer comprises a material having a refractive index nr substantially different from the refractive index nc of the cover stacking material in order to obtain sufficient reflection at the boundary between the reflective layer and the cover piling. For example, the reflecting layer is a 15 nm layer made of Al having a refractive index nr = 0.7-4.6i At 405 nm wavelength and a cover layer is made, for example, from a cured transparent material by UV or a polycarbonate film with a refractive index nc = 1.5 ... According to the standard for DB-ROM, the grooves are formed in a spiral pattern that has a track pitch of 0.320 + / - 0.010 μm. The length of the grooves in the direction of the runway is modulated according to a limited run length code with run lengths >; 2CBL and < 8CBL, where CBL = 80.00 nm +/- 0.07 nm or 74.50 nm +/- 0.07 nm. Figure 3 shows the installation for carrying out the step of depositing a reflective layer with a pattern that covers the surface by deposition of the inclined electrodeposition type of shadow electrodeposition of the manufacturing method according to the invention. The substrate is placed at an angle of inclination with respect to the electrodeposition target. The optimum angle of inclination is directly related to the depth of the groove structure in the substrate to be covered with a reflection layer with a pattern. In most cases, an angle between 20 and 80 ° is preferred. The inclination is selected in such a way that the reflecting layer is predominantly deposited on the spaces between the grooves, that is, on the surface surface area of the substrate. In a position of the substrate inclined with respect to the target subjected to electrodeposition, the deposited reflection layer becomes asymmetric due to the shadow effect. This is illustrated in the left image in figure 3. If the substrate is placed on the opposite side of the lens with the same angle of inclination, s a reflection layer with a similar asymmetric pattern is obtained, but in this case with a coverage of opposite layer. To obtain a symmetrical coverage of the substrate, the rotation of the substrate with respect to the electrodeposition objective is proposed. This results in a layer with a symmetrical pattern, as illustrated in the lower panel in Figure 3. In Figures 4a-4d a second method for obtaining a reflection layer with a pattern is illustrated. The duplicated substrate (Figure 4a) is provided with a layer via centrifugal coating. This layer needs to have a substantial mismatch in the refractive index with respect to the cover layer that is provided on the top of the disc for reading. The layer subjected to centrifugal coating is subsequently etched isotropically such that only the lower part of the groove is covered with the layer subjected to centrifugal coating. In this way, a layer with a reflection with a pattern is obtained as a result (see FIG. 4d). Suitable materials having a refractive index substantially different from that of the cover layer are, for example, phthahalocyanine dyes, cyanine dyes, azo dyes. Protective layers based on diazonaphthoquinone can also be etched in a controlled manner with NaOH or KOH develators. An isotropic UV illumination step can be applied to accelerate the etching of the photoprotective layer. Alternatively (not shown), it is possible to deposit an additional reflective layer on the spaces separating the grooves, on the layer subjected to centrifugal coating that covers the lower part of the grooves and on the side walls of the grooves, and subsequently eliminate the layer applied by centrifugal coating that covers the bottom part of the grooves, which includes the portion of the additional reflective layer that covers this layer subjected to centrifugal coating. The additional reflective layer can be deposited by normal electrodeposition methods or by the deposition method of the inclined electrodeposition type as described in figure 3. It is noted that in relation to this date, the best method known by the applicant to carry In practice, said invention is that which is clear from the present description of the invention.